Initial results of the GeoSTAR prototype (Geosynchronous Synthetic Thinned Array Radiometer)

A prototype of the Geostationary Synthetic Thinned Array Radiometer (GeoSTAR) is presented. GeoSTAR is a concept for a Y-array of correlation interferometers operating in bands from 50 GHz to 180 GHz which will for the first time provide high spatial resolution, continuous soundings of the earths atmosphere from geosynchronous orbit. This paper presents preliminary data from a small (24-element) 50-55 GHz prototype system which has been built under NASA's Instrument Incubator Program to demonstrate the basic technology and calibration techniques needed for the larger (300 element) spaceborne system. Images are synthesized by Fourier transform of interferometric data, and it is essential that the spatial response of each antenna in the array be well characterized. This has been achieved using a novel variant of a Potter horn antenna which minimizes embedding effects in a closely packed array. The array electronics consists of low-power MMIC receivers, built by JPL, and a high speed digital correlator, built by the University of Michigan. Calibration circuits include a phase-switched correlated noise reference which is injected behind the antenna array; circuitry to modulate the receiver noise temperature and gain; and local oscillator phase shifters which are used to negate correlator offsets and quadrature imbalances. An outline of the data processing is presented, along with the first images from this system